The invention relates to rotary-wing drones such as quadricopters or the like.
Such drones are provided with multiple rotors driven by respective motors that can be controlled in a differentiated manner so as to pilot the drone in attitude and speed.
A typical example of such a drone is the AR.Drone of Parrot SA, Paris, France, which is a quadricopter equipped with a series of sensors (accelerometers, three-axis gyrometer, altimeter), a front camera picking up an image of the scene toward which the drone is directed, and a vertical-view camera picking up an image of the overflown ground.
The documents WO 2010/061099 A2, EP 2 364 757 A1, EP 2 431 084 A1 and EP 2 497 555 A1 (Parrot SA) describe such a drone, as well as the principle of piloting the latter through a phone or a multimedia player having a touch screen and integrated accelerometers, for example a cellular phone of the iPhone type or a player or a multimedia tablet of the iPod Touch or iPad type (registered trademarks of Apple Inc., USA).
The drone is piloted by the user by means of signals emitted by the sensor detecting the device inclinations, wherein such inclinations are replicated by the drone: for example, to make the drone move forward, the user tilts his device about the pitch axis thereof, and to move the drone aside to the right or to the left, the user tilts said device with respect to the roll axis thereof. That way, if the drone is controlled so as to tilt or “dive” downward (inclination according to a pitch angle), it will move forward with a speed that is all the more high that the inclination angle is important; conversely, if it is controlled so as to “nose up” in the opposite direction, its speed will progressively slow down, then will invert, going back rearward. In the same way, for a control of inclination about a roll axis, the drone will lean to the right or to the left, causing a linear displacement in horizontal translation to the right or to the left.
The user has at his disposal other commands, which are displayed on the touch screen, in particular “climb/descent” (throttle control) and “right rotation/left rotation” (rotation of the drone about its yaw axis).
The invention more particularly relates to a navigation system allowing to know the position of the drone in an absolute coordinate system (Galilean) linked to the ground, for example a geographic terrestrial coordinate system. The position of the drone at a given instant is expressed by two horizontal position coordinates with respect to a known ground origin, and an altitude coordinate with respect to the ground level (supposed to be horizontal).
The GPS satellite-based global positioning systems are well-known navigation systems allowing to deliver such 3D position coordinates, with a very high accuracy.
The GPS technique, based on the reception of signals emitted by several satellites, are however inoperative in a closed environment, inside a building, due to the impossibility to suitably pick up these signals.
The problem of the invention is to allow the drone to localize itself with the desired measurement accuracy in such an environment where it is not possible to receive GPS signals, so as to be able to perform a flight in full autonomy, i.e. without communication with the outside to obtain position information.
This latter constraint eliminates in particular the localization systems of the motion capture type, where a set of cameras follows the position of the drone to be localized and determines by calculation, based on the picked-up images, an estimated position that is sent to the drone through a radio link. This system requires heavy technical means (cameras, calculation means) as well as a wireless link with the drone for the sending of information to the latter from the calculator. Hence, it is not an autonomous navigation system.
The object of the invention is to allow the drone to localize itself, without communication with the outside, based on the only signals delivered by a series of simple pre-existing sensors, in particular a low-resolution vertical camera. And this:                with a high accuracy, typically a centimeter accuracy;        without drift, which excludes a simple integration of the accelerometer signals produced by the inertial unit;        giving a position in an absolution coordinate system, independent of the drone: in particular, when several drones progress together in a same space, it is essential that this coordinate system is common to all the drones;        providing directly the position of the drone in the coordinate system, without requiring a previous calibration;        with a high refresh rate, typically of the order of 30 Hz;        robustly, without risk of perturbation by false detections, inconsistent values and various artefacts, or by signals coming from other drones progressing simultaneously close to each other.        
The thus-obtained absolute position information will be able to be used for various purpose, in particular for a control in position of the drone with compensation for the sensor-drift phenomena, the effects of wind or the displacements of the mass of air in which the drone progresses, etc., and any other information liable, in particular, to distort the indications coming from the inertial unit.